docs: def Fahien_Schriver(dp, voidage, vs, rho, mu, L=1):
      r'''Calculates pressure drop across a packed bed of spheres using a
      correlation developed in [1]_, as shown in [2]_. Second most accurate
      correlation overall in the review of [2]_.
  
      .. math::
          f_p = \left(q\frac{f_{1L}}{Re_{Erg}} + (1-q)\left(f_2 + \frac{f_{1T}}
          {Re_{Erg}}\right)\right)\frac{1-\epsilon}{\epsilon^3}
  
      .. math::
          q = \exp\left(-\frac{\epsilon^2(1-\epsilon)}{12.6}Re_{Erg}\right)
  
      .. math::
          f_{1L}=\frac{136}{(1-\epsilon)^{0.38}}
  
      .. math::
          f_{1T} = \frac{29}{(1-\epsilon)^{1.45}\epsilon^2}
  
      .. math::
          f_2 = \frac{1.87\epsilon^{0.75}}{(1-\epsilon)^{0.26}}
  
      .. math::
          f_p = \frac{\Delta P d_p}{\rho v_s^2 L}
  
      .. math::
          Re_{Erg} = \frac{\rho v_s  d_p}{\mu(1-\epsilon)}
  
      Parameters
      ----------
      dp : float
          Particle diameter of spheres [m]
      voidage : float
          Void fraction of bed packing [-]
      vs : float
          Superficial velocity of the fluid (volumetric flow rate/cross-sectional 
          area)[m/s]
      rho : float
          Density of the fluid [kg/m^3]
      mu : float
          Viscosity of the fluid, [Pa*s]
      L : float, optional
          Length the fluid flows in the packed bed [m]
  
      Returns
      -------
      dP : float
          Pressure drop across the bed [Pa]
  
      Notes
      -----
      No range of validity available.
  
      Examples
      --------
      >>> Fahien_Schriver(dp=8E-4, voidage=0.4, vs=1E-3, rho=1E3, mu=1E-3)
      1470.6175541844711
  
      References
      ----------
      .. [1] R.W. Fahien, C.B. Schriver, Paper presented at the 1961 Denver
         meeting of AIChE, in: R.W. Fahien, Fundamentals of Transport Phenomena,
         McGraw-Hill, New York, 1983.
      .. [2] Erdim, Esra, Ömer Akgiray, and İbrahim Demir. "A Revisit of Pressure
         Drop-Flow Rate Correlations for Packed Beds of Spheres." Powder
         Technology 283 (October 2015): 488-504. doi:10.1016/j.powtec.2015.06.017.
      '''
      Rem = dp*rho*vs/mu/(1-voidage)
      q = exp(-voidage**2*(1-voidage)/12.6*Rem)
      f1L = 136/(1-voidage)**0.38
      f1T = 29/((1-voidage)**1.45*voidage**2)
      f2 = 1.87*voidage**0.75/(1-voidage)**0.26
      fp = (q*f1L/Rem + (1-q)*(f2 + f1T/Rem))*(1-voidage)/voidage**3
      return fp*rho*vs**2*L/dp